Obesity, defined as an excess of body fat relative to lean body mass, is associated with numerous, important clinical and psychological morbidities, the former includes, but is not limited to: hypertension, elevated blood lipids, and Type II or non-insulin-dependent diabetes mellitus (NIDDM), and decreased life expectancy. The presence and degree of obesity are usually determined by reference to the absolute weight of an individual when compared to age and height matched ideals, or by reference to the individual's body mass index, that is, body weight (in kilograms) divided by height (in meters2), compared with age matched ranges. There are approximately 6-10 million individuals with NIDDM in the United States, including 18% of the total population over 65 years of age (see, e.g., Harris, et al., 1987. Int J Obes 11: 275-283). In addition, approximately 45% of males and 70% of females with NIDDM are obese, and their diabetes is substantially improved or even eliminated by weight reduction (see, e.g., Harris, 1991. Diabetes Care 14: 639-648). Both obesity and NIDDM are strongly heritable, though few of the predisposing genes have been identified. Hence, the molecular genetic basis of these metabolically related disorders is an important, poorly understood problem.
The assimilation, storage, and utilization of nutrient energy constitute a complex homeostatic system central to survival of metazoa. Among land-dwelling mammals, storage in adipose tissue of large quantities of metabolic fuel in the form of triglycerides is crucial for surviving periods of food deprivation. The need to maintain a fixed level of energy stores without continual alterations in the size and shape of the organism requires the achievement of a balance between energy intake and expenditure. However, the molecular mechanisms that regulate energy balance remain to be elucidated. The isolation of molecules that transduce nutritional information and control energy balance will be critical to an understanding of the regulation of body weight in health and disease.
Examination of the concordance rates of body weight and adiposity amongst mono- and dizygous twins or adoptees and their biological parents have suggested that the heritability of obesity (0.4-0.8) exceeds that of many other traits commonly thought to have a substantial genetic component, such as schizophrenia, alcoholism, and atherosclerosis (see e.g., Stunkard, et al., 1990. N Engl J Med 322: 1483-1487). Familial similarities in rates of energy expenditure have also been reported (see e.g., Bogardus, et al., 1986. Diabetes 35: 1-5). Genetic analysis in geographically delimited populations has suggested that a relatively small number of genes may account for the 30-50% of variance in body composition (see e.g., Moll, et al., 1991. Am. J. Hum. Genet 49: 1243-1255).
A major advance in understanding the molecular basis for obesity occurred with the cloning of the ob gene. The mouse obesity (ob) gene encodes an adipose tissue-derived signaling factor for body weight homeostasis. The mouse ob gene encodes a 4.5-kilobase adipose tissue mRNA (see, e.g., Montague, et al., 1997. Nature 387: 903-908) with a widely conserved 167-amino acid open reading frame and a 21-amino acid secretory signal sequence. The predicted amino acid sequence of the protein product of this gene, leptin (from the Greek leptos, meaning thin), is 84% identical in humans and mice and has features of a secreted protein (see, e.g., Zhang, et al., 1994. Nature 372: 425). Several recent studies have shown that recombinant ob protein purified from Escherichia coli can correct the obesity related phenotypes in ob/ob mice when exogenously administered (see e.g., Campfield, et al., 1995 Science 269: 546; Pellymounter, et al., 1995. Science 269: 540; Stephens, et al., 1995. Nature 377: 530). Weight-reducing effects of recombinant leptin were also observed in normal mice and mice with diet-induced obesity.
Interestingly, recent studies have suggested that obese humans and rodents (other than ob/ob mice) are not defective in their ability to produce leptin mRNA or protein and generally produce higher levels than lean individuals (see e.g., Maffei, et al., 1995. Nature Med 1: 1155; Considine, 1995. J. Clin. Invest. 95: 2986; Hamilton, et al., 1995. Nature Med 1: 953). This data suggest that resistance to normal or elevated levels of leptin may be important factors in human obesity.
Leptin is a 16 kDa hormone that is the afferent signal in a negative feedback loop regulating food intake and body weight. The leptin receptor is a member of the cytokine receptor family. Leptin's anorexigenic effect is dependent on binding to the Ob-Rb isoform of this receptor which encodes a long intracytoplasmic domain that includes several motifs for protein-protein interaction. Ob-Rb is highly expressed in the hypothalamus suggesting that this brain region is an important site of leptin action. Signal transduction by this class of receptor generally depends on ligand-induced phosphorylation of soluble tyrosine receptor kinases such as Janus kinase (JAK)-1, -2, -3, and tyk2. These kinases, in turn, phosphorylate tyrosine residues on the receptor which serve as docking sites for SH2 proteins. Phosphorylation of SH2 proteins, following receptor-binding, initiates signal transduction. Leptin binds to a homodimer of the Ob-Rb isoform of its receptor, thus activating JAK2. While Stat3-regulated transcription is activated by leptin in vivo, the identity of other components of this signal transduction pathway have not yet been quantitatively identified.
In recent studies, mutation of the mouse ob gene has been demonstrated to result in a syndrome that exhibits pathophysiology which includes: obesity, increased body fat deposition, hyperglycemia, hyperinsulinemia, hypothermia, and impaired thyroid and reproductive functions in both male and female homozygous ob/ob obese mice (see e.g., Ingalis, et al., 1950. J Hered 41: 317-318). As previously discussed, recent studies have suggested that obese humans and rodents (other than ob/ob mice) are not defective in their ability to produce leptin mRNA or protein and generally produce higher levels than lean individuals. Two distinct mutations of this gene have been identified. One mutant, designated SM/Ckc-+Dacob2J/ob2J, expresses no leptin mRNA (see, e.g., Lonnqvist, 1996. Q. J. Med. 89: 327-332); whereas the other mutation, designated C57BL/6J, overexpresses by 20-fold a mRNA species resulting from a single-base mutation in the ob gene at codon 105. This mutation changes the coding sequence for arginine (Arg)-105 in leptin to a premature stop codon, and causes production of a truncated inactive form of leptin (see, e.g., Zhang, et al., 1994. Nature 372: 425).
The mouse ob gene (GenBank Accession No. U22421) and its human homolog (GenBank Accession No. NM—000230) have been cloned (see, e.g., Zhang, et at, 1994. Nature 372: 425-432). The protein product of this gene, leptin, which has been postulated to act as a blood-borne hormone responsible for weight maintenance, is a 16-kDa plasma protein synthesized and secreted by adipocytes (see e.g., Halaas, et al., 1995. Science 269: 543-546; Pelleymounter, et at, 1995. Science 269: 540-543; Weigle, et at, 1995. J. Clin. Invest. 96: 2065-2070). The ob/ob mouse phenotype has been attributed to a deficiency in active leptin. A number of laboratories have reported that administration of recombinant leptin to C57BL/6J ob/ob mice, normal lean, or diet-induced obese mice resulted in weight loss through reduced food intake and increased energy expenditure (see e.g., Weigle, et al., 1995. J Clin Invest 96: 2065-2070; Barash, et at, 1996. Endocrinology 137: 3144-3147). Use of leptin or leptin receptor have been proposed as novel therapeutics in the treatment of human obesity. See, e.g., PCT Patent Applications WO 97/26335, WO 98/06752 and WO 99/23493. Other uses for leptin or leptin receptor include (i) diabetes (see, e.g., PCT Patent Applications WO98/55139, WO98/12224, and WO97/02004); (ii) hematopoiesis (see, e.g., PCT Patent Applications WO97/27286 and WO98/18486); (iii) infertility (see, e.g., PCT Patent Applications WO97/15322 and WO98/36763); and (iv) tumor suppression (see, e.g., PCT Patent Applications WO98/48831), each of which are incorporated herein by reference in their entirety.
Leptin fragments, and most particularly an 18 amino acid fragment comprising residues 57VTGLDFIPGLHPILTLSK74 taken from full length human leptin [the full length sequence is shown in SEQ ID NO:17], have been reported to function in weight loss, but only upon direct administration through an implanted cannula to the lateral brain ventricle of rats. See, e.g., PCT Patent Applications WO97/46585, which is incorporated herein by reference in its entirety. Those fragments in PCT Patent Applications WO97/46585 are different from the fragments of this invention.
To date, most leptin-related studies able to report weight loss activity from administration of recombinant leptin, leptin fragments and/or leptin receptor variant have administered said constructs directly into the ventricles of the brain. See e.g., Weigle, et al., 1995. J Clin Invest 96: 2065-2070; Barash, et al., 1996. Endocrinology 137: 3144-3147. Administration of any treatment directly into the brain has serious drawbacks for widespread use of such treatment in the human population. Only studies by the inventors have been able to show significant weight loss activity due to administered of leptin peptides by more favorable methods, namely, through intraperitoneally (i.p.) administration, to test subjects. See, Grasso et al., 1997. Endocrinology 138: 1413-1418.
High-affinity leptin binding sites have been identified in mouse choroid plexus, the leptin receptor (OB-R) gene has been cloned (GenBank Accession No. AF098792), and genetic mapping has localized this gene in the same interval of mouse chromosome 4 that contains the db locus (see, e.g., Tartaglia, et al, 1995. Cell 83: 1263-1271). Several transcripts of the OB-R, resulting from alternative splicing, have also been identified. Defects in OB-R produce a syndrome in the mutant diabetic db/db mouse that is phenotypically identical to the ob/ob mouse (see, e.g., Ghilardi, et al., 1996. Proc. Natl. Acad Sci. USA 93:6231-6235). In contrast to ob/ob mice, however, administration of recombinant leptin to C57BLKS/J-m db/db mice does not result in reduced food intake and body weight (see, e.g., Roberts and Greengerg, 1996. Nutrition Rev. 54: 41-49). Expression of murine leptin receptors has also been detected at high levels in non-neural tissues including the lung, kidney and ovary (see, e.g., Meier, 1996. Eur. J. Endocrinol. 134: 543-545; Chehab, et al., 1996. Nature Genet 12: 318-320). In adult humans, highest expression of OB-R is in heart, liver, small intestine, prostate and ovary, whereas lung and kidney express it at low levels (see, e.g., Cioffi, et al., 1996. Nature Med. 2: 585-589).
Expression of the ob gene in humans is highly correlated with body fat and body mass index, with greater expression observed in obese than in normal-weight individuals (see, e.g., Considine, et al., 1996. N. Engl. J. Med 334: 292-295). A similar correlation between serum leptin concentrations and ob mRNA levels in adipose tissue of obese individuals has also been found (see, e.g., Maffei, et al., 1995. Nature Med 1: 1155-1161). Because leptin concentrations are high in the serum of most obese humans, but decrease with weight loss, human obesity is believed to result from leptin resistance (see, e.g., Lonnqvist, et al., 1995. Nat. Med 1: 950-953; Misra and Garg, 1996. J. Invest. Med 44: 540-548). Furthermore, since obese humans do not have elevated cerebrospinal fluid levels of leptin, even though their plasma concentrations may be five-fold higher than nonobese individuals, the rate-limiting factor contributing to leptin resistance in obese humans appears to be related to defective leptin transport into the CNS (see, e.g., Banks, et al., 1996. Peptides 17: 305-311; Caro, et al., 1996. Lancet 348:159-161).
In humans, the ob gene is expressed almost exclusively in adipose tissue, and codes for a protein that is 84% homologous to mouse leptin (see, e.g., Considine, et al., 1995. J. Clin. Invest. 95: 2986-2988; Masuzaki, et al., 1995. Diabetes 44: 855-858). Recently, however, placental sources of leptin have been identified (see, e.g., Masuzaki, et al., 1997. Nat. Med 3: 1029-1022; Senaris, et al., 1997. Endocrinology 138: 501-4502). Although the ob gene and the OB receptor gene (db) are normal in most cases of human obesity, a frameshift mutation (see, e.g., Montague, et al., 1998. Int. J. Obes. 22: 200-205), and polymorphism in the 5′ non-translated region of the human ob gene (see, e.g., Considine, et al., 1996. Biochem, Biophys. Res. Commun. 220: 735-739; Hager, et a, 1998. Int. J. Obes. 22: 200-205) in a number of morbidly obese humans with low serum leptin concentrations have been reported. Possible linkage of extreme obesity to markers flanking the human ob gene, has also been proposed (see, e.g., Clement, et al., 1996. Diabetes 45: 687-690; Reed, et al., 1996. Diabetes 45: 691-694). These aforementioned recent findings suggest that administration of recombinant leptin, or leptin mimetics of even higher potency than leptin, may be possible approaches to the treatment of at least some forms of human obesity.
There is some evidence that leptin enters the brain via a saturable transport system. See, e.g., Banks et al., 1996, Peptides, 17: 305-311. Because the majority of obese humans do not have elevated cerebrospinal fluid (CSF) levels of leptin, even though their plasma levels may be five-fold higher when compared to nonobese individuals, the rate-limiting factor associated with leptin resistance in human obesity may be related to defective leptin transport into the central nervous system (CNS). See, e.g., Caro et al., 1996. Lancet 348: 159-161; Schwartz et al., 1996. Nat Med 2: 589-593. The ability of centrally administered leptin to reduce food intake and body weight gain in diet-induced obese mice resistant to peripherally administered leptin, is consistent with a mechanism of obesity which may involve saturated or defective leptin transport. See, e.g., Van Heek et al., 1997. J. Clin Invest 99: 385-390.
In this regard, the mature form of circulating leptin is a 146-amino acid protein that is normally excluded from the CNS by the blood-brain barrier (BBB) and the blood-CSF barrier. See, e.g., Weigle et al., 1995. J Clin Invest 96: 2065-2070. Thus, efforts to develop leptin-related peptide agonists of low molecular weight, or nonpeptide mimetics that can be transported across the BBB and blood-CSF barrier by mechanisms independent from those by which leptin is transported take on added importance. Identification of active epitopes within the leptin molecule, therefore, is important to the development of leptin analogs which can be administered peripherally, and thus have potential usefulness in the treatment of human obesity and its related dysfunctions.
In addition, there is a current need for methods and related compositions that could be utilized in detecting physiological obesity or other conditions related to abnormalities of the endogenous leptin pathway. Accordingly, given the aforementioned observations, there remains an as yet unfulfilled need for the development of low molecular weight, highly-potent peptide agonists of leptin, or nonpeptide leptin mimetics which are permeable to the Blood-Brain Barrier and can thus enter the central nervous system (CNS) without assisted-transport. The development of such pharmacophores may ultimately lead to novel methods of treatment for physiological obesity and/or other conditions which are related to abnormalities of endogenous leptin pathway, as well as a possible extension of their application to other obesity-related dysfunctions (e.g., Type II or non-insulin-dependent diabetes mellitus (NIDDM)).
The citation of any reference herein should not be deemed as an admission that such reference is available as prior art to the instant invention.